Astronomical tuning of the Aptian stage and its implications for age recalibrations and paleoclimatic events

The Aptian was characterized by dramatic tectonic, oceanographic, climatic and biotic changes and its record is punctuated by Oceanic Anoxic Events (OAEs). The timing and duration of these events are still contentious, particularly the age of the Barremian-Aptian boundary. This study presents a cyclostratigraphic evaluation of a high-resolution multiproxy dataset (δ13C, δ18O, MS and ARM) from the Poggio le Guaine core. The identification of Milankovitch-band imprints allowed us to construct a 405-kyr astronomically-tuned age model that provides new constraints for the Aptian climato-chronostratigraphic framework. Based on the astronomical tuning, we propose: (i) a timespan of ~7.2 Myr for the Aptian; (ii) a timespan of ~420 kyr for the magnetic polarity Chron M0r and an age of ~120.2 Ma for the Barremian−Aptian boundary; and (iii) new age constraints on the onset and duration of Aptian OAEs and the ‘cold snap’. The new framework significantly impacts the Early Cretaceous geological timescale.

1. It claims the chaotic transition from 2.4 myr to 1.2 myr at ca. 115.5 Ma. The evidence is very weak and purely based on the subjective recognition based on the low pass filtered output. And this claimed event doesn't have any theoretical support. Perhaps no existing astronomical solution is able to produce the claimed events, although it is still possible that none of the solutions is able to catch this event due to the chaos of the Solar system. In sum, this conclusion, without further evidence and robust evaluation, is not sufficient for Nat Commun, a top-tier journal in geosciences.
2. It also argues a time span of 7.1 myr duration for the Aptian. This duration is much shorter than that published by Huang et al., 2010, but generally similar to other recent calculations. However, data from the low-sedimentation interval -Selli -is incomplete. Thus the long eccentricity cycles marked as E13 to E17 are arbitrarily inferred, thus is not able to provide constraints for the Aptian duration. Therefore the third conclusion in the abstract cannot stand either.
3. The writing style of the abstract needs improvement. There is no background, no raised questions, nor no test hypothesis. Moreover, this Aptian cyclostratigraphy work cannot impact the geologic time scale from the Jurassic to the Early Cretaceous because the Aptian duration is not so long.
Taken together, this manuscript doesn't reach the quality for nature communications. However, it does have its merits and worth publishing in a good geology journal.
Reviewer #2: Remarks to the Author: The major claims of the paper are a new age estimate for the Barremian/Aptian boundary and new estimates of the timing and duration of key events of the Aptian including OAEs and cooling/warming cycles. The estimate for the B/A boundary is not entirely novel as very similar estimates were used in the 2020 GTS. But I think the community would appreciate the independent validation of the updated stage boundary age estimate presented in this study. Similarly, the estimates of timing and duration of Aptian events provide independent validation of competing age dates from previous studies, which is novel in that this method can mostly reproduce the ages produced from radiometric methods. Notably, they find the duration and timing of the late Aptian 'cold snap' to be consistent with other estimates of the timing of the chaotic transition and the duration of the event as constrained by bioand chemo-stratigraphic studies. This is of broad interest since it addresses emerging questions about the extent and degree of global warmth during the mid-Cretaceous and suggests a relationship between the cold snap and orbital parameters. These results are of an immediate interest to different geoscience subfields. This paper will influence thinking in the field because it demonstrates that astronomically tuned age models can reproduce radiometrically obtained dates, but it also furthers the use of these kinds of age models without consideration of sequence stratigraphy, which is a looming issue with the method. A major critique of the work centers around the disregard for sequence stratigraphy when interpreting the records of the core and especially when relating the study core to other localities. Sequence boundaries are a major problem for the application of astronomically tuned age models but could be first order detected through an evaluation of the impact of diagenesis on the isotope records. A scatterplot of oxygen and carbon data would help the reader assess the potential role of diagenesis in the records which could in turn help with the identification of sequence boundaries and flooding surfaces.
The study provides constraints on the duration and age of the Selli horizon (and other black shale horizons), but the authors do not discuss why sometimes they can replicate results from previous studies but at other times they are not be able to replicate the results. Is this sampling bias, preservation, or hiatus? A discussion of this would strengthen their arguments about fidelity in their age model. Another claim the authors make is that ages of OJP and HALIP suggest these incidents of volcanism are triggers of OAE1a. This claim is not well supported because there is no discussion on the age dates of HALIP nor is there a discussion on what defines OAE1a (is it the excursion, if so what part, or is it the bottom of the Selli level?). If the event is defined by different stratigraphic horizons at different localities then a discussion of how the authors define OAE1a is critical.
The work can be reproduced, but more information on the definition of particular stratigraphic horizons should be provided. Specifically, the paragraph made up of lines 454-469 is critically important and should be worked into the main text (perhaps the introduction) because it provides the reader with important information for evaluating the stratigraphic correlations among cores. Figure 2 should have information about the isotope zones names since they are referenced liberally throughout the text. It should be clearly stated if the carbon isotope data is from carbonate or organic matter. none of the solutions is able to catch this event due to the chaos of the Solar system. In sum, this conclusion, without further evidence and robust evaluation, is not sufficient for Nat Commun, a top-tier journal in geosciences.

Response:
The authors agree with reviewer 1 about the Comment 1, especially regarding the impossibility of an unequivocal inference of the ~ 2.4-Myr/1.2-Myr transition at ~ 115.5 Ma, the more that the chronostratigraphic framework provided for the PLG core is not supported by radioisotopic dating. Therefore, we completely removed such inference from the new version of our manuscript. Nevertheless, we would like to discuss some of the statements contained in this comment, on the basis of the following points: 1) There are a number of recent studies which pointed out that secular resonance transitions, resulting from the chaotic dynamical behaviour of the Solar System, may have had some implications in paleoclimatic events throughout the geological time. The first geological evidence for a geochronologically well constrained chaotic resonance transition during Cretaceous times was presented in a letter published in Nature (Ma et al., Nature 542, 468-470 (2017)). In this study, the authors indicated that a chaotic transition may have taken place during the OAE 3 (~ 84-88 Myr), by means of an integrated radioisotopic and astronomical timescale for North American Cretaceous records. Furthermore, they also stated that "... 2) It is also important to highlight that million-year-scale signals were clearly evidenced by the spectral analysis of the PLG core, as presented in our manuscript. As summarized below in Figure 1, a pervasive ~ 3.0-0.8 Myr-band is detectable for the four analyzed proxy datasets (MS, ARM, δ 13 C and δ 18 O), all of them above the confidence level (99.9%). Therefore, according to this evidence, as well as our agreement to discard the inference on the ~ 2.   1 Spectral analysis of PLG core data. 2π multitaper power spectra for a magnetic susceptibility (MS); b anhysteretic remanent magnetization (ARM); c δ 13 C; d δ 18 O data, with the AR(1) red noise spectral model and 85%, 90%, 95%, and 99% confidence levels (c.l.) for null hypothesis testing. The rectangle (dashed line) indicates the frequency range for the MSB (million-year scale band) -which was referred to "EAM/OAM" in Figure 3 and Supplementary Fig. 1 of the original version of our manuscript.

Comment 2:
It also argues a time span of 7.1 myr duration for the Aptian. This duration is much shorter than that published by Huang et al., 2010, but generally similar to other recent calculations.
However, data from the low-sedimentation interval -Selli -is incomplete. Thus the long eccentricity cycles marked as E13 to E17 are arbitrarily inferred, thus is not able to provide constraints for the Aptian duration. Therefore the third conclusion in the abstract cannot stand either.

Response:
We probably were not clear about the steps that guided us towards our proposition of the astronomical timescale (ATS) for the PLG core. The indication of the E13 -E17 long eccentricity range inferred for the Selli Level was not made arbitrarily.
We followed these steps: (1) due to the lack of radiometric ages for the Marne a Fucoidi Particularly regarding the Selli Level, it was not possible to directly estimate its mean duration from our cyclostratigraphic work, as the spectral peaks were badly resolved and the orbital-scale harmonic content was not "traceable" within this interval. Therefore, in order to avoid cyclostratigraphic misinterpretations, we chose to remove the data in this stratigraphic (ii) According to Kennedy et al. (2017) Ma, which is virtually the same age proposed by our age model (~120.2 Ma). Therefore, we are confident that our interpretation for the Selli Level duration is coherent, as well as that our cyclostratigraphic age model is consistent and provides a reliable chronostratigraphic interpretation for the entire PLG stratigraphy.

Comment 3:
The

Response:
We thank the reviewer for the extremely positive comments on this manuscript, and have done our utmost to address the points that have been raised.

Comment 1:
A major critique of the work centers around the disregard for sequence stratigraphy when interpreting the records of the core and especially when relating the study core to other localities. Sequence boundaries are a major problem for the application of astronomically tuned age models but could be first order detected through an evaluation of the impact of diagenesis on the isotope records. A scatterplot of oxygen and carbon data would help the reader assess the potential role of diagenesis in the records which could in turn help with the identification of sequence boundaries and flooding surfaces.

Response:
We followed the reviewer's suggestion and made the C-O isotopes biplots (see below

Response:
The early Aptian OAE 1a is the most prominent mid-Cretaceous Oceanic Anoxic Event, typified by worldwide deposition of thick organic-rich horizons. As mid-Cretaceous OAEs were often accompanied by intensive marine biotic crises, understanding the factors that We discussed about the link between HALIP, OJP and

Response:
We thank the Reviewer for the suggestion. We modified the figure according to your suggestions, including the isotope zones names, stated that the carbon isotope data is from carbonate and modified the text according to the nomenclatures used for the isotopic zones "The δ 13 C values vary significantly in the Fig. 2c Reviewer #1: Remarks to the Author: I am glad to see the very detailed response to comments from both reviewers. The response clearly addressed my concerns on the long eccentricity cycles and the duration of the Aptian. It is nice that the 2.4 myr -1.2 myr transition has been replaced by the "million-year scale band" because the previous argument needs a lot of solid evidence that is beyond the ability of this paper.
Here is another concern: this paper only used traditional cyclostratigraphic approaches on the detection of the long eccentricity cycles based on the cycle-ratio method and bandpass filtering, which can be subjective. Recent advances in cyclostratigraphy include statistical tuning approaches, such as ASM, COCO, timeOpt, and the evolutive version of these methods. Can you do a similar analysis and show the sedimentation rate map from both conventional methods (cycle-counting) and sliding window statistical tuning approaches (either eASM, eCOCO, or eTimeOpt). At least one analysis will definitely greatly enhance our confidence in the interpretation of this nice study.
Moreover, eFFT should be explained when it is first used in the paper.
Reviewer #2: Remarks to the Author: This manuscript presents multi-proxy records from a core that spans a geochronologically controversial time interval, and uses astronomical tuning to address geochronological uncertainties especially as they pertain to stage boundaries and ancient climatic events. The results generated new age and duration estimates that compliment previously published dates that used radiometric methods.
The methodology, data analysis, interpretation, claims and conclusions appear sound. Although, most major conclusions are tied to geochronology, they should be of interest to a wide range of earth scientists since they concern the geologic timescale. It is nice that the 2.4 myr -1.2 myr transition has been replaced by the "million-year scale band" because the previous argument needs a lot of solid evidence that is beyond the ability of this paper.

Response:
We are glad to know that we properly addressed this problem.

Comment 1:
Here is another concern: this paper only used traditional cyclostratigraphic approaches on the detection of the long eccentricity cycles based on the cycle-ratio method and bandpass filtering, which can be subjective. Recent advances in cyclostratigraphy include statistical tuning approaches, such as ASM, COCO, timeOpt, and the evolutive version of these methods. Can you do a similar analysis and show the sedimentation rate map from both conventional methods (cycle-counting) and sliding window statistical tuning approaches (either eASM, eCOCO, or eTimeOpt). At least one analysis will definitely greatly enhance our confidence in the interpretation of this nice study.

Response:
The authors are thankful for these very constructive suggestions. We agree that detection of the long eccentricity cycles based only on the cycle-ratio method and bandpass filtering can be subjective. Therefore, we provide in the newer version of the manuscript statistical tests based on the correlation coefficient (COCO) and its evolutionary variant (eCOCO) methods.
For the COCO/eCOCO tests, we aimed as follows: (i) to verify the optimal sediment accumulation rate (SAR) against null hypothesis significance levels (H0: no orbital forcing); and (ii) to evaluate the reliability of our astrochronological interpretation. Such tests were performed by removing different long-term trends with a 'lowess' smoother for three distinctive sliding windows (4, 6 and 8 m). Significance levels were estimated by using Monte Carlo simulation (5000 iterations) and tested sedimentation rates range from 0.05 to 3 cm/kyr with a step of 0.01 cm/kyr using the software Acycle v 2.4.1. The results are provided in Supplementary material "COCO/eCOCO tests" (Figs. 2-4).
The COCO/eCOCO results support our previous cyclostratigraphic interpretations for the PLG core, based on the ~405-kyr tuned age model for the long eccentricity low-pass filter output from MS data. Our sedimentation rate curve (Fig. 4a -main text) give a mean SAR of ~0.44 cm/kyr for the most of Aptian. This result is consistent with the COCO/eCOCO results (two peaks of 0.52 cm/kyr and 0.58 cm/kyr associated to a null hypothesis significance level lower than 0.001 - Supplementary Figs. 2, 3 and 4).
We discuss these results in the main text: "So far, the only estimate of mean sedimentation rate inferred for the entire Aptian (based on the PLG core) in literature is ~ 0.24 cm/kyr 32 .